Physical activity has important implications for reducing obesity as well as risks for various cancers. However, effective interventions for increasing and maintaining physical activity have remained elusive. The goal of the proposed research is to pilot test an intervention for increasing physical activity based on Self-Determination Theory (SDT). SDT posits that people who are more intrinsically or autonomously motivated to perform a behavior will more consistently engage in that behavior, experience greater enjoyment of the behavior, maintain the behavior for longer, and achieve greater psychological well-being as a function of that behavior. SDT has demonstrated that, compared to a controlling context in which individuals are simply ordered to follow instructions, providing an autonomy-supportive context in which individuals are offered choices, or in which their concerns are acknowledged promotes autonomous motivation. These effects have been demonstrated across ethnic groups and cultures. Health care providers, whether they are personal trainers or physicians, can thus impact individuals' motivation for health behaviors by supporting these needs. This approach has already proven to be effective in other health-related areas including smoking cessation and weight loss. The proposed research will test whether the way in which a personal trainer presents information to a client has differential effects on participants' autonomous motivation for physical activity, which will in turn predict exercise behavior and subsequent physical and psychological health outcomes. Participants will be randomly assigned to work with either an autonomy-supportive or controlling personal trainer via computer. Thus, the proposed research will test the causal role of an autonomy-supportive context in increasing physical activity. Participants will meet with their trainer via computer once a week for 6 weeks. Between weekly sessions, they will keep track of their exercise behaviors and experiences using Personal Digital Assistants (PDAs; e.g., Palm Pilot).